1. Field of the Invention
The present invention relates to a crane whose lowest boom of a boom can be used both for a tower crane and a normal crane.
2. Description of the Related Art
In general, in a crane whose lowest boom portion of the boom which consists of several boom portions is used both for the tower crane and the standard crane (hereinafter usually referred to as a normal crane), the respective minimum distances between spreaders (distance between upper and lower spreaders) Sa and Sb are determined when the boom is raised maximally while the tower crane is considered as a standard, and the distances Sa and Sb are set approximately equal both for the cranes.
However, the maximum boom raise angle of the boom depends on the respective cranes, and thus, the distance is longer for the tower crane, and shorter for the normal crane when the boom is laid on the ground.
Consequently, if an upper spreader is lowered on a rear surface of the boom, the spreader is lowered on a second boom exceeding beyond the lowest boom.
As a result, when the boom is disassembled for the tower crane, it is necessary to relocate the upper spreader which has bee lowered on the second boom to a spreader retainer of the lowest boom.
In addition, when the boom is assembled, it is necessary to relocate the upper spreader from the spreader retainer to a position on the second boom where a boom guyline can be connected.
For the relocation of the upper spreader, there are required employment of another crane, maintenance of a boom raising/lowering rope at a proper tension at the same time, and the like, and this operation is thus very troublesome. Consequently, overall work efficiency of the assembly/disassembly of the boom decreases.